The present invention relates to a flow sensor and more particularly to a heat transfer detection type flow sensor comprising a heating portion and a heat receiving portion (sensing portion).
Such a thermal-type flow sensor that comprises a substrate is known where on a heating element and a heat-receiving element are disposed at the upstream side and at the downstream side, respectively, in the direction of the gas flow to be measured. When gas flows along the substrate, it is heated by the heating element at the upstream side and then the heat is transferred to the heat receiving element at the downstream side on the substrate. Since the quantity of the transferred heat is proportional to the gas flow, the flow sensor can determine the amount of the gas flow.
However, in the application of the above-mentioned conventional type flow sensor, the following problems arise.
A divided flow of gas moving along the surface of the substrate, as compared with a divided flow of gas moving above the surface thereof may have decreased flow and a decreased rate of flow consequently has a low efficiency of heat transfer because of the effect of fluid viscosity and contact resistance at the boundary's surface.
On the other hand, the divided flow of gas moving above the substrate surface contributes to the flow's measurement by taking heat from the heating element and giving heat to the heat-receiving element, but it can't sufficiently transfer the heat from the heating element to the heat-receiving element because of the heat's dissipation during its movement between the elements. This divided flow has also a low efficiency in heat transfer.
Furthermore, since the heating element and the heat-receiving element are resisting the gas flow, the divided flow, apart from the substrate surface, is apt to disappear from the substrate surface. When the substrate surface is placed right-side up, there is less heat from the heating element transferred to the heat-receiving element owing to the heat's dissipation by convection (the upward movement of the gas). This may cause a decrease in the heat's transfer efficiency.